Answer:
Most likely this was failure of nondisjunction occurred during meiosis I resulted with 2 pairs of homologous chromosomes failed to separate. Explanation:
Simple explanation is that if nondisjunction occurs in meiosis II, there would be 3 types of cells; n-2, n-1 and n. And if nondisjunction occurs in meiosis I there would be two types of cells n+2 and n-2 (see figure). If the 2n = 36 than n should equal to the 18. There are two types of cells n +2 (20 chromosomes) and n-2 (16 chromosomes) so answer is "nondisjunction in meiosis I".
